Final answer:
The grammatically correct sentence is 'Ba a robala'
Step-by-step explanation:
The grammatically correct sentence using the present tense -a- is option C: Ba a robala.
In this sentence, the subject 'Ba' (they) is followed by the present tense marker 'a' and the verb 'robala' (sleep). The -a- marker indicates that the subject is performing the action in the present tense.
For example, in English, we can say 'They are sleeping' where 'are' functions as a present tense marker similar to '-a-' in Setswana.
Learn more about Present tense usage in Setswana